THE BASE OF THE BRAIN 395 



the two is the cochlear root, the ventral is the vestibular 

 root. 



The facial nerve is attached at the lower border of the 

 pons, just to the medial side of the acoustic nerve, by two roots 

 a large motor root, and a small sensory root (O.T. pars 

 intermedia) (Figs. 150, 151). The two roots unite in the 

 internal acoustic meatus. 



Nervus Abducens. The abducens is a small nerve which 

 emerges from the groove between the lower border of the pons 

 and the lateral part of the pyramid. It is flattened out near 

 its origin and a surface view of it in that region gives a 

 deceptive idea of its size (Figs. 149, 150). 



Nervus Trigeminus. The trigeminal is the largest of all 

 the cerebral nerves. It is attached to the side of the pons, 

 nearer its upper than its lower border, by two roots a large 

 sensory root and a small motor root, which are in a line 

 with the facial and acoustic nerves. The large, sensory root 

 (portio major) is composed of a great number of fila loosely 

 held together, but the small, motor root (portio minor) is 

 more compact, and it emerges antero-medial to the point at 

 which the sensory root enters the pons (Figs. 149, 151). 



Nervus Trochlearis. The superficial origin of the trochlear 

 or fourth nerve can be seen when the anterior part of the 

 superior vermis of the cerebellum is displaced backwards. 

 It emerges from the anterior medullary velum, on the dorsal 

 aspect of the brain-stem, immediately below the quadri- 

 geminal lamina. It is a slender nerve and it has a long 

 intracranial course. In the first part of its course it winds 

 round the lateral side of the pedunculus cerebri, between the 

 cerebrum and cerebellum, to reach the interpeduncular region 

 (Figs. 149, 150). 



Nervus Oculomotorius. The oculo-motor nerve issues, by 

 several fila, from the sulcus oculomotorius on the medial 

 face of the cerebral peduncle in the interpeduncular fossa 

 (Figs. 149, 150). 



Nervus Opticus. The optic nerve is a large round nerve 

 which joins the antero-lateral angle of the optic chiasma 

 (Fig. 149). 



Nervi Olfactorii. The olfactory nerves, about twenty in 

 number on each side, arise in the nasal mucous membrane. 

 They pass into the cranium through the cribriform lamina of 

 the ethmoid, and they terminate in the olfactory bulb. It is 



